The invention relates to a measuring apparatus, in particular to a line delay measuring device, for location of cable faults in electrical cables, comprising a closed box-like housing and situated in the housing a voltage supply generating high tension which is applied to an image tube arranged within the housing, as well as operating elements arranged on the front board, such as switches, press buttons, indicator elements and connector sockets.
The high degree of mechanisation in mining operations requires a considerable utilisation of electrical cables and wiring. The preponderant reason for the breakdown of an electrical conductor is mechanical destruction. Sources of failure discovered in this connection were:
erroneous length PA1 conductor breaks PA1 low-ohmic connections PA1 short-circuits PA1 irregularities caused by alteration of the structure, of the dielectric and the like. PA1 location of a broken wire, PA1 short circuits, PA1 low-ohmic shunts, PA1 location of a damp cable section, PA1 location of junctions, PA1 length defects, PA1 loading inductances, PA1 line capacitors, PA1 interrupted or open metal casing, PA1 interruption or short-circuit in the cable comprising 2 insulations. PA1 1. Construction of a sheet steel housing, PA1 2. Division of the housing into two chambers, forming an intrinsically safe and a specially protected section. PA1 3. Potentiometers for high voltages (focus, intensity) are installed internally and are then not directly accessible from the outside, PA1 4. spindles of potentiometers and of rotary selectors for the pulse velocity (V/2) are extended by plastics material spindles and led to the outside through the intrinsically safe housing chamber via special bushings, PA1 5. the operating keyboards are replaced by miniature relays which are operated via an intrinsically safe circuit by means of press button keys, the position of the press button keys being indicated by light-emitting diodes, PA1 6. the rotary selector position for the pulse velocity is indicated digitally, PA1 7. battery monitoring by means of LED indication, PA1 8. digital display and light-emitting diodes for accumulator monitoring are situated behind a fused silica pane within the intrinsically safe space, whereas the display tube is situated in the specially protected chamber filled with quartz granulate. PA1 9. the housing screws are recessed and covered by casting compound.
Even in the case of intrinsically safe installations, the investment costs in respect of the cabling material are raised considerably with long-distance supplies and consequent requirement for high cable cross-sections. Furthermore, in the case of mining operations, standby conductors are kept in readiness so that a new cable line may be utilised immediately in case of breakdown of a conductor. Conductor sections which have been taken out of service because of defects which are not detectable from the outside, are normally, as a matter of principle, left in the drifts, after completion of the working operations in said drifts. It is also worth observing that the electrical departments of mines continuously employ operatives in overhauling and re-utilisation of disassembled cables and conductors. Use is made of conventional measuring apparatus such as ohm meters and transit flow testers, for this purpose, and a reutilisation of the whole cable section is abandoned in the case of cable defects which are not detectable by means of the measuring instruments specified, or if no external damage is visible.